1. Field of the Invention
The present invention relates to an image forming apparatus for forming color images using a plurality of image bearing members and an intermediate transfer belt or a transfer material conveying belt, and in particular, to such apparatus in which, in a color image formation, a plurality of image bearing members are in touch with the above belt, and in which, in a monochrome image formation, image bearing members not concerned with image formation are separated from the belt.
2. Related Background Art
In recent years, there has been an increasing demand for longer service life in image forming apparatus based on electrophotographic system. In an image forming apparatus disclosed in Japanese Patent Application Laid-Open No. H6-258914, four image bearing members are in touch with an intermediate transfer belt in a color-image formation mode. On the other hand, in a monochrome-image formation mode, image bearing members-on which toner images in color rather than in black are formed are separated from the intermediate transfer belt.
Similarly, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-108045, four image bearing members are in touch with a transfer material conveying belt in a color-image formation mode. On the other hand, in a monochrome-image formation mode, image bearing members on which toner images in color rather than in black are formed are separated from the transfer material conveying belt. In both image forming apparatuses mentioned above, image bearing members not in use are separated from the intermediate transfer belt and the transfer material conveying belt in the monochrome image formation mode to extend a service life of the image bearing members.
In the aforementioned image forming apparatus, however, switching of the image formation modes changes the number of image bearing members in contact with the intermediate transfer belt and the transfer material conveying belt. Change in the number of image bearing members in contact with them influences bias when the intermediate transfer and transfer material conveying belts are rotated. For this reason, bias needs controlling until the toner images on the image bearing members are transferred to the transfer material borne by the intermediate transfer belt or the transfer material conveying belt after a mode has been switched. The control of bias requires the time during which images cannot be formed, causing a problem in that productivity in image forming apparatus falls.